Display and lighting systems based on organic light emitting devices (OLEDs) have a variety of applications. The construction of OLED light sources and OLED displays from individual OLED elements or devices is well known in the art. A light-emitting layer of a luminescent organic solid, as well as optional adjacent semiconductor layers, is sandwiched between a cathode and an anode. The light-emitting layer may be selected from any of a multitude of fluorescent organic solids. The light-emitting layer may consist of multiple sub-layers or a single blended layer.
When a potential difference is applied across the anode and cathode, electrons move from the cathode to the optional electron-injecting layer and finally into the layer(s) of organic material. At the same time, holes move from the anode to the optional hole-injecting layer and finally into the same organic light-emitting layer(s). When the holes and electrons meet in the layer(s) of organic material, they combine, and produce photons. The wavelength of the photons depends on the material properties of the organic material in which the photons are generated. The color of light emitted from the OLED can be controlled by the selection of the organic material, or by the selection of dopants, or by other techniques known in the art.
Different colored light may be generated by mixing the emitted light from different OLEDs. For example, white-emitting OLED-lighting systems can be generated by applying a continuous down-conversion layer on the light emitting side of a blue OLED. The down-conversion layer comprises of a color changing material, for example phosphor particles.
In a typical OLED, either the anode or the cathode is transparent in order to allow the emitted light to pass through. If it is desirable to allow light to be emitted from both sides of the OLED, both the anode and cathode can be transparent.
The basic OLED has a structure in which an anode, an organic light emitting layer, and a cathode are consecutively laminated, with the organic light emitting layer sandwiched between the anode and the cathode. Generally, electrical current flowing between the anode and cathode passes through points of the organic light emitting layer and causes it to luminesce. The electrode positioned on the surface through which light is emitted is formed of a transparent or semi-transparent film. The other electrode is formed of a specific thin metal film, which can be a metal or an alloy.
Generally, phosphor has been used as a color conversion layer (CCL) in the fields of display and lighting applications such as light emitting diodes (LEDs), OLED, and the like. In particular, the use of phosphor in down conversion may be used to create white light. The combination of the light emitted from the phosphor material and the unabsorbed light from a blue emitting layer has been used considerably in white LED and OLED. However, such phosphor undergoes the reducing of luminous efficiency and the drifting of color coordinates due to heat and junction temperature when it is applied to high-power white LED. As such, the thermal stability of the phosphor has an important effect on the optical properties as the major luminescent material of white LED and OLED. In addition, phosphor may generate a yellowish color when the device is in an off-state due to absorption of ambient light and conversion of white color into yellow. This phenomena is undesirable from the aesthetic point of view.
These and other shortcomings of the prior art are addressed by the present disclosure.